The present invention generally relates to an apparatus for electronically sensing an averaged temperature. More specifically, the present invention relates to a temperature sensor which uses multiple, spaced sensing elements such as thermistors in a circuit, such as for use in monitoring an average air temperature across an area within a large ventilation (HVAC) system.
Controlled, forced-air ventilation systems are known which move air within buildings. In many ventilation systems, fans draw fresh outside air into a building, and exhaust stale interior air to the outside. The ventilation systems are used with venting or ducts to provide an air flow path throughout the building, including to and from heaters and/or air conditioners. Often the ventilations systems perform heat transfer (recovery) between the interior air to be exhausted and the outside air being introduced. For proper control of these ventilation systems, parameters such as fan speeds or damper positions are set and changed based upon sensed air temperatures within the building or within the system. Particularly in systems where air of different temperatures mixes, it is important to be able to accurately determine average air temperature, such as the average air temperature across a vertical cross-section at a location within a duct.
As explained in U.S. Pat. Nos. 6,592,254 and 6,890,095, incorporated by reference herein, early structures for sensing average temperatures included capillary tubes and resistance temperature detectors (“RTDs”) such as platinum strand sensors, and metallic tube-enclosed thermistor-based sensors. None of these sensors were adequately easy to install and robust for use as desired in many HVAC duct sensing environments.
The metallic tube-enclosed thermistor-based sensors in particular had problems. Ascertaining the location of the thermistors within the tube was difficult. Particularly for long runs of measurement (typically from six to twenty four feet or more), the tube was bulky and difficult to ship. Bending the tube improperly can cause inadvertent crimping and/or kinking of the metal, which could effectively sever the electrical connections or which could lead to small holes forming in the tube. Where small holes in a metal tube are created, cycled temperature differences can result in condensation on the inside of the tube which sometimes can affect the accuracy of the temperature sensor. Condensation at the location of a thermistor could short-circuit the thermistor and lead to anomalous temperature readings. The solder connections are exposed to tension and stresses associated with adjusting and bending the wires. Over time, the solder points weaken and electrical connections break. The resulting open circuit may be difficult to locate if the wire is placed inside a tube, and may be costly to repair no matter how the sensor is situated. The metallic nature of the tube requires dielectric insulation to prevent electrical shorting between the thermistors and the wall of the tubing. The insulation/metal tube support and protection configuration thermally insulates the thermistor or platinum strand from the air, slowing the response time of the averaging temperature sensor.
Whether each thermistor or its overlying insulation contacted the metal tube was inconsistent and depended upon installation. If the metal tube was bent in a particular fashion during installation, a thermistor might make solid contact with the metal tube for good thermal conductivity. If the metal tube was bent in a different fashion during installation, an air gap might exist between the thermistor and the metal tube retarding heat transfer therebetween. Thus, the amount of thermal conduction from the metal tube to each thermistor varied in inconsistent and unknown ways.
Assembly of the electrical circuit of thermistor arrays has been problematic. An insulative card has been used, allowing solder points between the leads for the thermistor to the wires extending between thermistor locations. The soldering card further adds thermal ballast to slow response time. Response time in the control systems is fairly significant, because delays in control can lead to damage to system elements, particularly if the system manipulates outside air at a drastically different temperature than the inside air.
Despite the plethora of problems noted here, the metallic tube-enclosed thermistor-based sensors became a market leading standard in the HVAC industry. In contrast to the metallic tube-enclosed thermistor-based sensors, the sensor described and claimed in U.S. Pat. Nos. 6,592,254 and 6,890,095 has begun to revolutionize averaging duct temperature sensors in the HVAC industry. Still, improvements can be made to averaging duct temperature sensors, particularly for certain environments of use.